A bandpass filter receives signals of a specified frequency band and attenuates the signals outside the specified frequency band. With popularization of mobile communication and advance of wireless network technology, the bandpass filter, which can capture a given range of signals and exclude unnecessary noise, has been a critical element in mobile devices.
Among the conventional bandpass filters, a Taiwan patent No. 1381574 disclosed a “Dual-Band Bandpass Filter”, which comprises a first resonator, a second resonator, a third resonator and a fourth resonator. A signal received by the first resonator is transmitted along the following two paths: in a first path, the signal is resonated by the first resonator and the second resonator and output by the second resonator; in a second path, the signal is resonated by the first resonator and a portion of the third resonator, then resonated by a portion of the third resonator and a portion of the fourth resonator, then resonated by a portion of the fourth resonator and a portion of the second resonator, and then output by the second resonator.
However, the first, second, third and fourth resonators of the conventional bandpass filter are wider and scarcely curved. Thus, the conventional bandpass filters have larger area. The mobile electronic devices (such as mobile phones and tablet computers) are growing more and more slim and lightweight to meet the requirement of consumers and the trend of the market. Therefore, developing a smaller-area bandpass filter has become an important subject in the related industry.